[Desktop] "Check for updates" spinner does not resolve Linux

**Description**

On Linux desktop installs (deb/rpm/flatpak), clicking Check for Updates in Settings → App leaves the spinner running infinitely ("Checking for updates..."). The check itself completes instantly on the main process with a null result and no error, but the renderer never transitions out of the checking state.

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**Steps To Reproduce**
1. Install the official desktop package on Linux (Ubuntu 24.04 aarch64, goose_1.48.0_arm64.deb)
2. Open Settings → App
3. Click Check for Updates

**Observed Behavior**
* Spinner keeps going indefinitely
Log signature (from ~/.config/Goose/logs/main.log):

```[2026-08-29 01:46:19.193] [info] === MANUAL UPDATE CHECK INITIATED ===
[2026-08-29 01:46:19.193] [info] Current version: 1.48.0
[2026-08-29 01:46:19.194] [warn] APPIMAGE env is not defined, current application is not an AppImage
[2026-08-29 01:46:19.194] [info] === MANUAL UPDATE CHECK COMPLETED in 1ms ===
[2026-08-29 01:46:19.194] [info] Auto-updater checkForUpdates result: null
```
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**Expected behavior**
Check either reports "up to date" (when on latest), or detects an available update (via the GitHub fallback) and offers to download/install. Either way the user should get some sort of feedback in < 10s. Checking for updates should be fast on anything but the slowest connection.

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**Screenshots**
If applicable, add screenshots to help explain your problem.

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**Please provide the following information**
- OS & Arch: Ubuntu 24.04.4 LTS, arm64 (aarch64) & Pop!_OS 24.04 LTS
- Interface: UI (Electron desktop)
- Version: v1.48.0 (also observed on v1.46.0 and v1.45.0)
- Extensions enabled: n/a
- Provider & Model: n/a
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**Additional context**
Add any other context about the problem here.

Do not begin implementation until the issue reaches **Ready** on the [Goose Issues board](https://github.com/orgs/aaif-goose/projects/1).

Look at the auto-updater logic in the main process, likely in a file handling updates for Linux packages (deb/rpm/flatpak). The logs show the check completes but the renderer state doesn't update. Start by finding where the update check result is sent back to the renderer and why the transition fails. Check the IPC communication between main and renderer processes. Verify the UI component in the settings page that shows the spinner. Running the app in debug mode and checking console logs will help trace the flow.

Written by the indexing model from the issue text.